The present invention concerns a pneumatic radial tire using fiber cords of polyolefin ketone for a belt cover layer and, more particularly a pneumatic radial tire for improving the high speed endurance and the road noise.
Along with the increase in quality and speed of recent vehicles, it is demanded to reduce the road noise during the travel and, furthermore, to improve the tire high speed endurance. In order to respond to such demands, it is proposed to use fiber cords of the polyolefin ketone presenting a predetermined strength and elastic modulus for the belt cover layer to be disposed around the outer peripheral side of the belt layer.
However, in the case of using the fiber cords of the polyolefin ketone for the belt cover layer, the elastic modulus provided by the fiber cord was not exploited sufficiently, and it was not satisfactory in terms of road noise improvement level. Namely, the polyolefin ketone fiber having a relatively low breaking elongation compared to nylon fiber or the like can hardly follow the lift deformation during the vulcanization. Consequently, the cord was broken or bitten easily, and it was impossible to maximize the elastic modulus of the fiber cord in a tire.
It is an object of the present invention to provide a pneumatic radial tire capable of improving the high speed endurance and the road noise considerably.
In order to achieve the aforementioned object, the pneumatic radial tire of the present invention is a pneumatic radial tire having a carcass layer mounted between a pair of right and left bead portions and a belt layer embedded in the outer peripheral side of the carcass layer in a tread portion, wherein a belt cover layer comprising the fiber cords made of the filaments of polyolefin ketone having a structure represented by the following formula (1) disposed around the outer peripheral side of the belt layer, and an elongation rate of the fiber cords composing the belt cover layer is equal or inferior to 2.5% in the tire.
xe2x80x94(CH2xe2x80x94CH2xe2x80x94CO)nxe2x80x94(Rxe2x80x94CO)mxe2x80x94xe2x80x83xe2x80x83(1) 
where, 1.05xe2x89xa7(n+m)/nxe2x89xa71.00,
R is an alkylene group having three or more carbons.
Thus, the disposition of the belt cover layer using fiber cords of polyolefin ketone satisfying specific conditions around the outer peripheral side of the belt layer, and the elongation rate of the fiber cords composing the belt cover layer made equal or inferior to 2.5% in the tire allow to deploy maximally the elastic modulus provided by the polyolefin ketone fiber cords, and thereby improve considerably the high speed endurance and the road noise compared to the prior art.
In the present invention, in order to obtain a sufficient effect improved in the high speed endurance and the road noise, the belt cover layer is the one where a ribbon material comprising at least one of the fiber cords arranged and coated with rubber is wound continuously at substantially 0xc2x0 in respect to the tire circumferential direction and, preferably, strength of the fiber cords composing the belt cover layer is equal or inferior to 0.77 g/D (0.68 cN/dtex) when elongated by 1.0%, and an elastic modulus thereof is 50 to 170 g/D (44.1 to 150.0 cN/dtex) when elongated by 2.0%. On the other hand, a breaking elongation of the fiber cords composing the belt cover layer is preferably equal or superior to 4%. It should be appreciated that the aforementioned fiber cords are applicable not only to the belt cover layer but also to the belt layer.
In the present invention, though denier is used as thickness of the fiber cord, the denier can be converted into decitex based on the relation of 1D =1.111 dtex.